In various processes, including petroleum and petrochemical processing, metallic bellows expansion joints are utilized to absorb relative movement in piping and conduit systems. The specific alloy utilized for the bellows depends on the service environment encountered in the particular process unit. In many cases, austenitic type steels and stainless steels are required due to the combination of elevated temperatures and corrosive elements in the process fluid. These austenitic steels and stainless steels are subject to metallurigal attack known as stress corrosion cracking under certain conditions. In some cases, such stress corrosion cracking is caused by the presence of certain types of acid-forming compounds such as hydrogen sulfide being present in the process stream and therefore on the bellows steel. For expansion joints operating at high temperatures, conditions conducive for the initiation of stress corrosion cracking often exist only during a shut-down of the process unit when the expansion bellows cool from their high operating temperature down to ambient temperature. Expansion joint bellows are particularly vulnerable to this type of failure due to the extremely high stress levels present in the bellows during such down time and the relatively thin material employed in bellows fabrication.
In practice, it has been found that the actual attack and initial cracking of a stainless steel metal bellows occurs only after the stainless steel has been sensitized, so to speak, by having been heated to a high elevated temperature and then cooled, and is then attacked by some kind of acid. A family of weak acids is formed on the stainless steel by combination of water and oxygen from the air and almost any sulfur compound, or product of sulfur corrosion, from the chemical process, when the metal bellows are cooled. When the acid forms on the sensitized metal, cracks will occur in the metal.
One way of combatting this phenomenon in the past upon unit shut-down has been to spray, or attempt to spray, a chemical solution through the expansion joint external shroud seal openings in order to try to flush the outside of the expansion bellows to neutralize any acid that may form on the exterior surface of said bellows. Attempts have also been made to spray a neutralizing solution on the interior surface of the conduit in an attempt also to flush the inner walls of the bellows. These attempts have proved difficult in application, or impossible, due to the configuration of the surrounding parts in the conduit to protect the bellows from direct exposure to the elevated temperatures of the process fluid. In addition, access to the internal surface of the bellows must be obtained from inside the expansion joint, and the time delay in gaining this access can be sufficiently long so as to permit cracking before such access is obtained. After exposure to the heated process conditions, this stress corrosion cracking can occur in a very short period of time after shut-down of the unit, and experience has shown that expansion joint failures will occur when proper treatment is not provided within hours of unit shut-down.
It has also been found that if such precautions are not taken for the prompt protection and flushing of an expansion bellows immediately after unit shut-down, the likelihood of stress corrosion cracks appearing is greatly increased but they will often be found only upon restart of the unit, at which time, when the failure is found, the unit must be again withdrawn from service at great cost and expense merely to replace the failed expansion joint.